Hot reducing refers to a process by which a steel tube (or pipe) of given diameter is reduced to a tube of smaller diameter by passing through a continuous series of roll stands, usually of two or three rolls per stand, each roll set having smaller nominal diameter than the preceding stand, without internal support to the tube and with the tube being reduced, having been preheated to an elevated temperature. As the outside and inside diameters of the tube are reduced, the tube length is elongated an amount related to the overall difference between entering and exiting cross sectional areas. To permit proper material flow, each successive roll stand rotates at higher speed than its preceding stand since each stand must accept and process in a given period of time a longer length of material than its predecessor. It is the relationship and control of these successive roll stand speeds that is the subject of this invention.
In the early days of tube and pipemaking, these roll stand speeds were designed to match the elongation of tube length based on the diameter reduction of each stand. In other words, the speed of each successive stand was increased only enough to match the surface speed of the roll to the surface speed of the tube or pipe passing through it. It was soon found that in this type of design, and due to the radial forces imposed on the tube to cause it to reduce, part of the material flow was radial instead of entirely longitudinal. The inside diameter was reduced more than the outside diameter, thereby causing the tube or pipe wall thickness to increase as diameter reduction progressed. This wall increasing phenomena limited the scope of application of the process because the wall thickness increased as the overall (outside) diameter reduction increased. The resulting effect was that with large reductions of outside diameters, the circular shape of the inside diameter was distorted due to non-uniform radial forces around the tube section caused by non-uniform rolling forces applied across the grooves of an essentially round pass formed by two or three rolls. Theoretically, only with an infinite number of rolls forming the pass would rolling forces be applied uniformly around the tube section. As a result, with large diameter reductions and particularly with heavier entering wall thicknesses, the inside diameter was distorted to a square shape in the case of two-roll mills and to a hexagonal shape in three-roll mills.
To overcome these shape problems and to extend the working range of the reducing process, mill designers began to calculate and design roll speeds of successive stands to be slightly higher than those required to match tube surface speed with roll surface speed at the normal roll groove pitch line diameter, in an attempt to develop a slight tension between stands so as to cause a greater portion of tube area reduction to move longitudinally into elongation and a smaller portion to move radially into wall thickness increase. This speed increase above normal or "overspeed" did not cause rolls to slip on the tube surface but merely caused the roll groove pitch line, or neutral point, to adjust to a new position of pitch diameter which again matched tube surface speed This practice worked well for the particular tube section for which it was designed, but the amount of tension required between stands varies with the amount of cross sectional area reduction. In those early days, reducing mills were usually driven by lineshafts from a single motor and the individual roll stand speed increase was effected through fixed gearing from the lineshaft to the roll stand. The speed increase between stands was therefore fixed and had to be calculated as a compromise over the total product range, again limiting the extremes to which the process could be applied.
It obviously became necessary to design speed control systems whereby the tension developed in the tube between stands could be varied depending on the tube section being processed. Individually driven roll stands, each with its own electric drive motor, provided the flexibility for such control. Initially, each stand drive motor had its own speed control and speeds were manually set to achieve a calculated speed increase curve. Today, these drives are computer controlled to accurately maintain speed regulation to preset speed curves calculated for each individual tube section being processed. Hydraulic and mechanical differential drives have been developed to accomplish the same result. These mills came to be known as stretch reducing mills because of the high tension forces they are capable of developing in the tube between roll stands.
In the modern stretch reducing mill, sufficient interstand tension can be developed to not only eliminate the natural tendency of tube wall thickness to increase, but to cause the wall thickness to decrease, resulting in even greater elongation than outer diameter reduction alone would provide.
One serious disadvantage of the stretch reducing process was found to be that this interstand tension could not be fully developed until several stands of the mill were in contact with the tube or pipe to provide sufficient grip or traction to maintain the desired tension. Therefore the tube ends were not subjected to the same tension as the intermediate tube body. As a result, both the front end of the tube entering the mill and the back end exiting the mill reacted in the same manner as the early reducing mills, with the wall thickness increasing until sufficient tension was generated to reduce the thickness of the main portion of the tube. This end effect was great enough to cause the end portions to be out of tolerance for heavy wall thickness over a considerable length of the tube. The difference between end and body wall thickness is related to the amount of tension or stretch necessary to produce the desired body wall thickness. A disadvantage is that sufficient tension is required to maintain entering wall thickness, without decreasing or increasing it, such that out-of-tolerance end thickness is necessarily produced. This disadvantage of stretch reducing rendered the process useless to those manufacturers who could supply only short lengths of tube or pipe to the stretch reducing mill. It was not uncommon for the out-of-tolerance lengths, or crop ends, to be ten to fifteen feet long at each end when area reductions were high. Obviously, if entering material was limited to 40 to 50 feet of length, the resulting yield losses could not be tolerated.
In an effort to reduce this end effect, various systems of crop end control were developed. Some of these systems are very complex and expensive, particularly in the case of the individual electric drive mills. The principle of operation of each of the various forms of crop end control is the same. A speed change is made from the normal speed curve as the tube ends are entering or leaving the mill. The intent of the speed change is to increase the speed differential between roll stands as the ends are being processed and to return to the normal speed curve for the main portion or body of the tube. By increasing the speed differential, tension in the tube ends is built up more quickly to attempt to reduce the length of out-of-tolerance product. Each of the various crop end control systems has proven to be only partially effective. The length of crop ends has been somewhat reduced, but none has eliminated the problem completely. A negative effect of present systems is that as the tube ends progress through the mill with increased speed differential, the portions of the tube body adjacent to the ends is also subject to increased tension during the time the roll stand speeds are adjusted back to normal. The wall thickness of these portions is thereby reduced more than desired with the possibility that those portions can be out-of-tolerance for light wall thickness.